A mask ROM, in which data is written in a memory cell by means of an ion implantation with mask pattens, has reasonable properties applicable to a larger capacity and a lower cost-per-bit so that it is advantageous in manufacturing mass products by using the mask patterns for data writing (or programming) and also in enhancing the efficiency of occupation with memory cells each of which is composed of one transistor per bit. Current applications for the mask ROM are typically addressed in storing data for fonts, characters and fixed programs (e.g., BIOS program) in personal computers, word processors, electronic entertainment apparatuses, printers, electronic organizers, personal digital assistants (PDA).
As there is an usual limit upon the operating speed for accessing data in a mask ROM, being effected mostly by resistance and capacitance strayed along word lines and bit lines therein, it is necessary to reduce a delay rate on a word line and to accelerate the time for sensing data. Although several efforts have been proposed to reduce the delay factors, such as using policide as a material for a word line, and increasing the number of divided blocks in a cell array and so on, the operating frequency in a mask ROM has still been positioned, with a remarkable gap, under those in general processors (e.g., MPU or CPU).